1. Field of the Invention
The present invention relates to a high frequency surface acoustic wave device and, more particularly, to a high frequency surface acoustic wave device which may be manufactured by the same manufacturing equipment, and with the same material, as those required for manufacturing a low frequency surface acoustic wave device.
2. Description of Related Art
The surface acoustic wave device has been widely used as the middle-frequency filter of a TV. Besides, as the improvement in the material technology, the surface acoustic wave device can be used as a filter in the mobile communication field. Moreover, since the surface acoustic wave device has certain characteristics such as low loss level, high attenuation, small-size, and light-weight, the surface acoustic wave device is applied widely in the wireless communication field.
Since the surface acoustic wave device is widely applied in the communication products, which transmit or receive signals in a high frequency band, the demands on the high frequency surface acoustic wave device are thus increased dramatically. For the lithium niobate (LiNbO3) substrate widely used in the surface acoustic wave device, the acoustic velocity of the surface acoustic wave thereof is 3295 m/s. Therefore, if the surface acoustic wave device is going to operate at 1800 MHz, the line widths of the surface acoustic wave filter must be as narrow as 0.5 μm or below. However, the contact-type lithography apparatus cannot be used in the manufacturing process with such high resolution. Instead, a stepper and an expensive dry-etching process must be used to manufacture such a high frequency surface acoustic wave device on the lithium niobate substrate. As a result, the cost of the manufacturing process is increased dramatically and not favorable for massive production process.
Furthermore, a manufacturing process including depositing a diamond layer or a diamond-like layer on a piezoelectric substrate, such as the lithium niobate substrate, the lithium titanate substrate, or the quartz substrate, is proposed. However, the roughness of the diamond film of the surface acoustic wave device manufactured by the above process is not suitable for the transmission of the surface acoustic wave. Therefore, a planarization process, such as the chemical mechanical planarization (CMP) process, is required. In other words, the proposed manufacturing process has lots of drawbacks, such as high cost, high technical threshold, and the requirement of a post-manufacturing process.
Other manufacturing processes have also been proposed, such as forming a high acoustic velocity layer on a low-cost silicon substrate having a piezoelectric layer thereon, but since the quality control on the piezoelectric layer is not as easily achieved as that of the piezoelectric substrate, so the manufacturing process cannot be employed in mass production yet. Besides, due to the peeling of the diamond-like layer, the surface acoustic wave device manufactured by depositing a diamond-like layer on a piezoelectric substrate is not stable enough for in-field operation.
Therefore, a peeling-free, low cost, easy to manufacture device having higher acoustic velocity, surface acoustic wave device is required in the communication industry.